Fibronectin is a large protein (Mr about 250,000 daltons) that mediates the interaction of cells with the extracellular matrix. The molecule exists as a disulfide linked dimer that is elongated and flexible and participates in a complex set of macromolecular interactions in tissue that influence cell adhesion and migration. This well-studied matrix protein consists of multiple repeating modules of approximately 40-90 amino acids that can be classified into three types of homology units: types I, II and III. Clusters of these modules constitute individual fully active domains of the multifunctional molecule. Protein crystallography will be used to study the molecular basis for two biological roles of fibronectin: cell adhesion and matrix assembly. Type III modules are structural building blocks in the domains that mediate these two processes. Module III/10 contains an arginine-glycine-aspartic acid (RGD) sequence recognized by the alpha5beta1 cell surface adhesion receptor (integrin). Module III/1 binds to the type I modules at the amino- and carboxyl regions of fibronectin in a self-association mode prior to the formation of extracellular matrix. A fragment of module III/1 (III/1-C) binds to fibronectin (module III/11) and enhances fibronectin adhesiveness. The normal process of matrix assembly is disrupted in tumorigenic cells, and the lack of a stable accumulated matrix may influence the invasive properties of cancer cells. Recombinant fragments of fibronectin (i.e., including III/10, III/1, III/1-C and III/11 modules) as well as multimodule fragments (i.e., III/10-11, III/8- 10, III/8-11) will be crystallized. Also, module IIICS that exists in alternatively spliced variants of human fibronectin and is recognized by the alpha4beta1 integrin will be studied. Atomic models derived from these crystallographic analyses will be used to understand cell attachment and matrix assembly defects that permit tumor invasion and metastasis and to begin to construct a supra-structural image of fibronectin considering interactions of adjacent as well as distant modules.